In the pharmaceutical industry, it is important to know the parameters which characterize the tableting properties of powders which are compressed to form tablets used for human and animal consumption. The bonding index of the powder is probably the most important parameter, and this index is determined from fundamental considerations of specific tablet models in order to determine how the interparticle bonds form. With respect to the experimental tests which are carried out on these models, these typically involve an indentation hardness test wherein a ball is impacted against one face of the model, and a tensile strength determination wherein the model is subjected to transverse compression.
To create the test model or compact, the latter being an enlarged blocklike tablet which has a square cross section approximately 19 mm on a side and a thickness somewhat less than the side dimension, it has been conventional to utilize a press mechanism having a split die assembly into which the powder is positioned, and then subjected to uniaxial compression to form the test tablet. In this known press mechanism, which has been used by the assignee of this application for many years, the die has a die cavity of square cross section, and is split diagonally into two die halves. A first pressure cylinder imposes a compression force on the die halves in a horizontal direction which is perpendicular to the vertical plane which splits the die halves. A second pressing cylinder has a punch thereon of a cross section so as to be vertically insertable into the die cavity and exert a vertical compression force on the powder therein to create the test tablet. The two pressure cylinders are joined to a common pressure source, the latter being a manual pumping arrangement, so that pressure builds up simultaneously in the two cylinders in a controlled but fixed ratio relationship to one another. After the powder has been suitably compacted, the two pressure cylinders are simultaneously depressurized, again in a controlled but fixed ratio relationship to one another, through a suitable dump valve which permits the pressure fluid to flow back to the reservoir. This effectively results in what is known as "triaxial decompression" of the test tablet, following the uniaxial compression thereof. This triaxial decompression is necessary so that the resulting test tablet is free of fracture lines and is suitable for tensile strength testing.
In this known press mechanism, a stationary punch is positioned at the lower end of the die cavity in opposed relationship to the movable punch. This lower punch, in selected situations, has a retractable spring-urged pin projecting upwardly therefrom for forming an axial orientation hole through the center of the compact.
This press mechanism is utilized for creating compacts for both tensile strength testing and impact testing.
While this known press mechanism has provided suitable test compacts for a number of years, nevertheless there has been a continuing desire to improve the manner in which the test compacts are formed so as to permit more accurate evaluation of the bonding properties of the material, particularly by measurement of the impact and tensile strength. There has been a desire to improve not only the ease and efficiency of making the test compacts, but also the ability to significantly increase the uniformity of the properties of the compacts so that the compact will be free of fracture lines and other undesired irregularities and nonuniformities which can influence or effect the test measurements which subsequently result from the impact and tensile strength tests.
Accordingly, it is an object of this invention to provide an improved press for creating enlarged blocklike tablets or compacts from a powdered material, which press significantly improves upon the performance characteristics of the known press described above.
More specifically, in the improved press of this invention, the press has an improved split die assembly which can be readily opened and closed and, in cooperation with the bottom punch, greatly facilitates both the loading of powder into the die assembly and the removal of the formed compact therefrom. The improved press also incorporates structure, specifically a horizontally shiftable carriage mechanism which mounts the die assembly thereon, so that the die assembly can be readily horizontally moved between a loading and unloading position wherein it is spaced sidewardly a substantial distance from the pressing region so as to greatly increase convenience of access thereto, and a pressing position wherein the die assembly is aligned with the pressing cylinder to permit exertion of a pressing force on the powder within the die assembly to create the compact.
The press also has an improved pressing mechanism which, while it again includes two perpendicularly oriented pressing cylinders cooperating with the die assembly, also has selectable and independent contols which permit precise and independent regulation of the pressure in each of the cylinders in a coordinated manner both during compression and during decompression particularly for permitting precise and adjustable control over the individual pressures in the two cylinders during the critical triaxial decompression stage to hence adjustably control the relationship between these pressures.
Other objects and purposes of the invention will be apparent to persons familiar with presses of this general type upon reading the following specification and inspecting the accompanying drawings.